The output waveform of a video digital analog convertor (DAC) may contain current changes or steps that cause radiation of electromagnetic waves. This radiation can interfere with other communication and control signals in the same RF bands. One method of reducing such unwanted emissions is by reducing the harmonic content of the video signal.
Many attempts to address problems of this nature have been tried in the past. These attempts include utilizing transistors responsive to control signals for applying a reverse biasing potential to a reference voltage transistor and a digital input transistor at a rate such that there is little surge current in the reference transistor when the DAC is switched between its sleep mode and its regular operating mode. Such a circuit is shown in U.S. Pat. No. 5,184,129, issued Feb. 3 1993, to Fung et al.
Similarly in U.S. Pat. No. 5,023,614 also issued to Fung et al, there is taught the use of a control transistor responsive to control signals for applying a reverse bias to a reference voltage transistor in each stage at a rate such that the rate of change of current in the reference voltage transistor is less than a predetermined magnitude.